1. Field of the invention
The invention concerns the acquisition and digitization in three dimensions of the shape of any object by means of systems comprising an optical sensor with a source of laser radiation and one or more cameras scanning the trace of the laser beam on the object under study.
2. Description of the Prior Art
A technique of this kind is described in detail in the documents FR-A-2 627 047, FR-A-2 642 833 and FR-A-2 629 198.
To be more precise, this technique generates a "laser plane", that is to say a sectoral lamellar beam which is very thin but whose width covers all of the object to be scanned, with one or two cameras viewing this plane at two different angles of incidence. The system is placed at the end of a mobile manipulator arm, the arm of a numerically controlled machine tool for example, to scan the laser plane over the part to acquire the surface of the part in three dimensions progressively. The scanning may be achieved by pivoting or by movement in translation of the sensor relative to the part or by keeping the sensor fixed in position and moving the part relative to it, in which case the part is mounted on a remote controlled multi-axis table, for example.
The sensors used in this technique until now have been relatively bulky, fragile and complex.
One object of the invention is to propose a sensor structure enabling the sensor to be significantly miniaturized whilst retaining or even increasing its accuracy and its resolution (the "resolution" being the pixel size of the system and the "accuracy" allowing for the digitization and reconstitution stages; it will be shown later that the accuracy of positioning can be better than one pixel given various processing operations carried out during the laser trace analysis stage).
The benefit of a miniaturized sensor, apart from its general convenience, is that it can be used in even a highly restricted space, for small parts, on surfaces that are difficult of access, etc, in other words whenever great accuracy is required in a reduced area.
However, in this case the sensor is much closer to the part to be scanned and this gives rise to a number of new problems, especially that of the depth of field, which decreases as the sensor moves closer to the object.
At present He-Ne laser sensors are adjusted with a focus about one meter from the source and therefore with significant "backoff" (which is indispensable in any event given the relatively large size of the helium-neon laser and its various associated units). As used herein, the term "backoff" refers to the distance between the laser sensor and the object to be scanned.
Combined with a long focal length, this large backoff can produce an extremely fine trace with a large depth of field, typically a lamellar beam less than 0.2 mm thick over a depth of field of 100 mm.
What is more, as the lamellar beam is produced by static means (usually a cylindrical lens), the spread of the beam in combination with the large backoff produces a relatively low local energy density (in the order of 1 .mu.W/mm.sup.2), which means that the beam is not hazardous to the operator.
On the other hand, although it is possible to miniaturize the sensor (in particular by using a laser diode in place of the He-Ne laser), if it is not possible or desirable to set back the sensor from the object the depth of field problem arises because of the focussing at a much shorter distance (typically 10 cm rather than 1 m), with a corresponding effect on the thickness of the trace and therefore on resolution and accuracy.
Because the energy density varies in inverse proportion to the distance, it increases very significantly to the point where the beam is hazardous should it impinge on the eye of an operator near the object.
An object of the invention is to retain the advantages of current sensors despite miniaturization and the much smaller physical backoff relative to the object.
The basic principle of the invention is to create a virtual backoff of the source within the sensor to procure conditions that are substantially the same as those for a long focus beam, so compensating the small physical backoff due to the configuration of the sensor-object system.